Although Friday's massacre at Sandy Hook Elementary School has prompted renewed calls for reinstating the federal "assault weapon" ban, we know for a fact that such a law would not have stopped Adam Lanza or made his attack less deadly, because it didn't. The rifle he used, a .223-caliber Bushmaster M4 carbine, was legal under Connecticut's "assault weapon" ban, which is similar to the federal law that expired in 2004. Both laws, in addition to listing specifically prohibited models, cover semiautomatic rifles that accept detachable magazines and have at least two out of five features: 1) a folding or telescoping stock, 2) a pistol grip, 3) a bayonet mount, 4) a grenade launcher, and 5) a flash suppressor or threaded barrel designed to accommodate a flash suppressor. The configuration of the rifle used by Lanza, which his mother legally purchased and possessed in Connecticut, evidently was not covered by that definition.